1. Field of the Invention
This invention relates to an optical system which, in particular, magnifies an image while changing an object distance in an imaging optical system using a solid-state image sensor.
2. Description of Related Art
In recent years, an electronic endoscope using a small-sized CCD (solid-state image sensor) has come into prominent use. According to this endoscope, a diseased part in a human body can be observed through a TV monitor by many people, and thus observations and diagnoses by a plurality of doctors are possible. Moreover, there is the great advantage that a patient is examined while viewing his diseased part. Of such endoscopes, special attention is devoted to a so-called nearby-object observable endoscope which is used for the purpose of closely viewing the diseased part and observing its microstructure to diagnose the extent of penetration of a minute diseased part or possible cutting areas. Some of optical systems for nearby-object observable endoscopes are disclosed, for example, in Japanese Patent Publication No. Sho 61-44283 and Japanese Patent Preliminary Publication No. Hei 4-218012. Each of these optical systems, which are the magnifying optical systems of three-lens-unit type and four-lens-unit type, can bring about a high observation magnification, but has the problem that the entire length of the optical system is considerable and a great burden is imposed on a patient.
In contrast to this, each of optical systems disclosed in Japanese Patent Publication No. Sho 55-15004 and Japanese Patent Preliminary Publication No. Hei 1-279219 is such that the entire length of an optical system is reduced by using the optical system of two-lens-unit type. Such an optical system, however, has the drawback that a working distance (which is hereinafter abbreviated to WD) where a nearby object is observed is long and a sufficiently high magnification is not obtained. There is also the drawback that a field angle 2w in the observation of an object located at a long distance (which is hereinafter referred to as the distant-object observation) is as small as 100xc2x0 or less and thus much time is required to find a diseased part. In a single-focus endoscope for screening that has no function of the observation of an object located at a short distance (which is hereinafter referred to as the nearby-object observation), the field angle 2w is about 130xc2x0, and hence it is desirable to have a wide field angle of the same extent as this.
The optical system of two-lens-unit type facilitates a reduction in the entire length, but when a converging lens unit is moved along the optical axis, ray heights in the distant- and nearby-object observations fluctuate considerably, and thus it becomes difficult to suppress the fluctuation of astigmatism. Hence, in accordance with the movement of the converging lens unit, a meridional image surface changes largely.
In FIG. 1A, reference numeral 1 represents a ray in the distant-object observation and 2 represents a ray in the nearby-object observation. As seen from this figure, the ray 1 passes through high positions of a first diverging lens unit L1 and a second converging lens unit L2. In this state of the ray 1, if powers of the first diverging lens unit L1 and the second converging lens unit L2 are balanced to reduce the production of astigmatism to zero, the function of the first diverging lens unit L1 will be impaired and astigmatism will be undercorrected when the ray height is lowered by the ray 2. That is, with the conventional optical system of two-lens-unit type, the stroke of the movement of the converging lens unit cannot be increased due to correction for astigmatism. In this way, it becomes difficult to reduce the WD in the nearby-object observation to increase the magnification of the optical system.
The short stroke of the movement of the converging lens unit is also related with the fact that a field angle in the distant-object observation cannot be increased. It is for this reason that the size of the field angle in the distant-object observation has a correlation with that in the nearby-object observation, and when the field angle in the distant-object observation is increased, the magnification of the optical system in the nearby-object observation is decreased, but when the amount of stroke is small, a reduction of magnification in the nearby-object observation cannot be obviated.
It is, therefore, an object of the present invention to provide an optical system for nearby-object observable endoscopes which is reduced in the entire lent and has a wide field angle as in a single-focus endoscope for screening and at the same time, has a high magnification in the nearby-object observation.
In order to achieve this object, the optical system for endoscopes includes, in order from the object side, a first lens unit, a second lens unit, and a third lens unit. The second lens unit is movable. This optical system is such that the movement of the second lens unit allows astigmatism due to the first lens unit and the second lens unit to be corrected and astigmatism due to the second lens unit and the third lens unit to be corrected.
Further, the optical system for endoscopes of the present invention includes an image-forming system comprising, in order from the object side, at least, a first diverging lens unit with a negative function, a second converging lens unit with a positive function, and a third diverging lens unit with a negative function. This optical system allows the second converging lens unit to be moved along the optical axis and satisfies the following conditions:
xe2x88x921.3 less than xcex22T less than xcex22W
xe2x80x831 less than |xcex23T|
where xcex22T is the magnification of the second converging lens unit in the nearby-object observation, xcex22W is the magnification of the second converging lens unit in the distant-object observation, and xcex23T is the magnification of the third diverging lens unit in the nearby-object observation.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.